Our research program aimed at understanding the functions of the RAG1-RAG2 protein complex in molecular detail has progressed further in the past year. In 2015, we reported the first crystal structure of the RAG1/RAG2 complex. This was a major advance, making it possible for the first time to give an explanation of the so called 12/23 rule in V(D)J recombination, which forces it to work on two different DNA sites. But this was a structure of the protein complex alone, without DNA, which left many questions about mechanism unanswered. Thus it has been essential to obtain detailed information on structures containing DNA, and to study them at all stages of the cleavage reaction. As the DNA cleavage occurs in several defined steps, at least four distinct species of the RAG-DNA complex can be obtained. Crystals of most of these species have now been obtained and their structures are in the process of being refined. We are also extending our work toward more full-length versions of the RAG proteins, which contain regulatory elements controlling RAG activity.